1. Field of the Invention
This invention concerns an oil recovery process, and more specifically a surfactant oil recovery process. Still more specifically, this invention is concerned with an oil recovery process using a surfactant which is stable in formations whose water contains high concentrations of salt and/or divalent ions such as calcium and magnesium, and which formations additionally have a temperature in excess of 120.degree. F.
2. Description of the Prior Art
Petroleum is normally recovered from subterranean formations in which it has accumulated by penetrating the formations with one or more wells and pumping or permitting the petroleum to flow to the surface through these wells. Recovery of petroleum from formations is possible only if certain conditions exist in the formations. The petroleum must be present in the formation in an adequately high concentration, and there must be sufficient permeability or interconnected flow channels within the formation to permit the flow of fluid therethrough if sufficient pressure is applied to the fluid. When the formation has natural energy present in the form of an underlying active water drive, or gas dissolved in the petroleum which can exert pressure to drive the petroleum to the producing well, or a high pressure gas cap above the petroleum within the formation, this natural energy may be utilized to recover petroleum. Recovery of petroleum by utilization of natural energy is referred to as primary recovery. When this natural energy source is completed, or in those instances where the formation does not contain sufficient natural energy to support primary recovery, some form of supplemental or enhanced recovery process must be applied to the formation in order to extract petroleum therefrom. Supplemental recovery is sometimes referred to as secondary or tertiary recovery, although in fact it may be primary, secondary or tertiary in sequence of employment.
Water flooding, which involves the injection of water into the subterranean, petroliferous formation for the purpose of displacing petroleum toward the producing well, is the most economical and widely practiced supplemental recovery method. Water does not displace petroleum with high efficiency, however, since water and oil are immiscible, and also because the interfacial tension between water and oil is quite high. Persons skilled in the art of oil recovery have recognized this inherent weakness of water flooding and many additives have been described in the literature for decreasing the interfacial tension between the injection water and the formation petroleum. For example, U.S. Pat. No. 2,233,381 (1941) discloses the use of polygylcol ether as a surface active agent or surfactant to increase the capillary displacement efficiency of an aqueous flooding medium. U.S. Pat. 3,302,713 discloses the use of petroleum sulfonate prepared from a specified boiling range fraction of the petroleum feed stock for a surfactant in oil recovery operations. Other surfactants which have been proposed for oil recovery operations include alkylpyridinium salts, alkyl sulfates, alkylaryl sulfates, ethoxylated alkyl or alkylaryl sulfates, alkyl sulfonates, alkylaryl sulfonates, and quaternary ammonium salts.
The above described surfactants are satisfactory in some instances, particularly in formations wherein the salinity as well as the water hardness, i.e., concentration of divalent ions including calcium and magnesium is relatively low, usually below a value of from about 200 to about 500 parts per million. When the formation water salinity or hardness exceeds the tolerance level of petroleum sulfonate, for example, mixtures of surfactants such as a mixture of anionic and nonionic surfactants are sometimes utilized. U.S. Pat. Nos. 3,811,504; 3,811,505; 3,811,507, describe certain mixtures of specific anionic and nonionic surfactants which exhibit satisfactory performance in petroleum formations containing unusually high concentrations of divalent ions, e.g. from about 500 to about 18,000 parts per million calcium and magnesium.
While the above described surfactant mixtures may be effective in surfactant recovery operations under ideal conditions, there are problems associated with the use of these materials in some petroleum containing formations. For example, the passage of an aqueous solution containing two totally different species of surfactant dissolved therein frequently results in the selective adsorption of one material rather than the other, or the adsorption of one of the materials at a different rate than the other. Since the optimum performance of a multi-component surfactant system is achieved only when the various surfactant species are all present in critical concentrations, this shift in concentration as a result of selective adsorption of surfactant can result in there being non-optimum or even inoperative concentrations of surfactants at certain points in the formation.
Another problem which frequently degrades the performance received from surfactant flooding operations is associated with the formation temperature. Petroleum sulfonates as well as other alkyl or alkylaryl sulfonates are relatively stable at temperatures normally encountered in subterranean petroleum containing formations; however, these materials are usually not effective in the presence of high salinities and/or high formation water hardness. Conversely, nonionic surfactants such as polyethoxylated alkylphenols are effective for surfactant flooding in formations containing high salinity water or hard water, but these materials become insoluble at temperatures in the range of from about 100.degree. to about 125.degree. F, referred to as their cloud point. Thus, while the materials are not degraded permanently, they are removed from aqueous solution and therefore are ineffective so long as the temperature is above their cloud point.
A type of anionic surfactant which is frequently effective for use as a co-surfactant in combination with petroleum sulfonates or alkyl or alkylaryl sulfonates is a water soluble salt of a polyethoxylated alcohol or alkylphenol sulfate. Unfortunately, the sulfate linkage causes the molecule to be highly sensitive to temperature, which causes hydrolysis or other permanet degradation of the molecule to a non-reactive form, and so these surfactants cannot be used in formations above a certain critical temperature which is characteristic of particular surfactants being used. For example, the sulfate salt of a polyethoxylated alkyphenol havng an alkyl chain length of about 9 carbon atoms and having approximately four or five ethylene oxide groups per molecule is degraded above about 140.degree. F. Since the surfactant solution will ordinarily be present in the formation for long periods of time, e.g. in terms of a number of months or even years, the thermal stability of a surfactant solution is extremely important.
In view of the foregoing discussion, it can be appreciated that there is a substantial, unfulfilled commmercial need for a surfactant usable in oil recovery operations which surfactant is tolerant of formation water salinity and hardness and which can tolerate temperatures in excess of 120.degree. F for long periods of time without hydrolyzing or becoming insoluble.